1. Field of the Invention
The present invention relates to the noninvasive monitoring of arterial blood pressure, and more particularly to the noninvasive monitoring of arterial blood pressure, including systolic pressure, diastolic pressure, mean pressure, pulse rate, and pressure waveform characteristics, under high motion conditions.
2. Description of the Related Art
Various different methods may be used to measure blood pressure: invasive, oscillometric, auscultatory, tonometric, and sensor-based. The invasive method, which is known as an arterial line (A-Line), involves insertion of a needle into the artery and is generally accepted as the “gold standard.” The other methods are noninvasive. The oscillometric method determines blood pressure from the amplitude of pressure oscillations in a pressurized cuff, typically measured within the cuff while the cuff is slowly deflated. The auscultatory method involves monitoring Korotkoff sounds as an inflated cuff placed around a cooperating artery of the patient slowly deflates. Systolic pressure is indicated when Korotkoff sounds begin to occur, while diastolic pressure is indicated when the Korotkoff sounds become muffled or disappear. The tonometric method typically uses an array of pressure sensitive elements which have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is to be measured. The array is pressed against the site to measure a reference pressure directly from the wrist, which is correlated with arterial pressure.
The oscillometric, ausculatory, and tonometric methods have not been entirely satisfactory. Because both the oscillometric and the auscultatory methods require inflation of a cuff, they are not entirely suitable for performing frequent measurements and measurements over long periods of time. The frequency of measurement is limited by the time required to inflate and deflate the cuff, and the pressure imposed by the cuff is uncomfortable to the patient. Moreover, both the oscillometric and auscultatory methods lack accuracy and consistency. While the tonometric method eliminates the need for a cuff, accurately positioning and maintaining the individual pressure sensitive elements over the underlying artery is difficult. The tonometric method requires that the system be calibrated to compensate for gain, which is the ratio of pressure outside the artery to the pressure inside the artery. Improper placement will make calibration ineffective, and patient movement during measurement will change the gain and affect the accuracy of the measurement.
Various sensor-based noninvasive blood pressure (“NIBP”) monitoring approaches that overcome the disadvantages of the invasive, oscillometric, auscultatory and tonometric methods have been developed by Medwave, Inc. of Danvers, Mass. Some of these approaches are described in the following United States patents: U.S. Pat. No. 5,450,852 entitled “Continuous Non-Invasive Blood Pressure Monitoring System” which issued Sep. 19, 1995 to Archibald et al.; U.S. Pat. No. 5,640,964 entitled “Wrist Mounted Blood Pressure Sensor” which issued Jun. 24, 1997 to Archibald et al.; U.S. Pat. No. 5,642,733 entitled “Blood Pressure Sensor Locator” which issued Jul. 1, 1997 to Archibald et al.; U.S. Pat. No. 5,649,542 entitled “Continuous Non-Invasive Blood Pressure Monitoring System” which issued Jul. 22, 1997 to Archibald et al.; U.S. Pat. No. 5,720,292 entitled “Beat Onset Detector” which issued Feb. 24, 1998 to Poliac; U.S. Pat. No. 5,722,414 entitled “Continuous Non-Invasive Blood Pressure Monitoring System” which issued Mar. 3, 1998 to Archibald et al.; U.S. Pat. No. 5,738,103 entitled “Segmented Estimation Method” which issued Apr. 14, 1998 to Poliac; U.S. Pat. No. 5,797,850 entitled “Method and Apparatus for Calculating Blood Pressure of an Artery” which issued Aug. 25, 1998 to Archibald et al.; U.S. Pat. No. 5,941,828 entitled “Hand-Held Non-Invasive Blood Pressure Measurement Device” which issued Aug. 24, 1999 to Archibald et al.; U.S. Pat. No. 6,159,157 entitled “Blood Pressure Measurement Device with Sensor Locator” which issued Dec. 12, 2000 to Archibald et al.; U.S. Pat. No. 6,241,679 entitled “Non-Invasive Blood Pressure Sensing Device and Method using Transducer with Associate Memory” which issued Jun. 5, 2001 to Curran; U.S. Pat. No. 6,558,335 entitled “Wrist-Mounted Blood Pressure Measurement Device” which issued May 6, 2003, to Thede; U.S. Pat. No. 6,589,185 entitled “Method and Apparatus for Calculating Blood Pressure of an Artery” which issued Jul. 8, 2003, to Archibald et al.; and U.S. Pat. No. 6,695,789 entitled “Disposable Non-Invasive Blood Pressure Sensor” which issued Feb. 24, 2004, to Thede et al. As described in these patents, blood pressure is measured from pressure waveform data that is acquired non-invasively while the degree of compression of an artery is being varied. A sensor has a contact surface that includes a terminus of a pressure transmission path and a terminus of a conformable wall. The compressible wall is located adjacent to and ideally surrounds the pressure transmission path, which is effectively isolated from forces within the conformable wall. The contact surface of the sensor is positioned on the surface of an anatomical structure over the artery and a varying pressure is applied. Suitable anatomical structures include the wrist, the inside elbow, the ankle, and the top of the foot. Pressure waveforms that result from arterial blood pressure and the varying holddown pressure propagate through the pressure transmission path and are sensed by a transducer to produce pressure waveform data. The pressure waveform data is analyzed to determine waveform parameters which relate to the shape of the sensed pressure waveforms, and one or more blood pressure values are derived based upon the waveform parameters. The wall applies pressure to the artery while at the same time suppressing force in a direction generally parallel to the artery from being applied to the pressure transmission path.
While blood pressure measurements typically are made under quiescent conditions, there are circumstances under which pressure measurements must be made under high motion conditions. The high motion conditions may be imposed by external events, such as motion during emergency patient gurney or ambulance transport, or by the patient as when the patient is talking, eating, walking, or running on a treadmill. Regardless of the source, the high motion conditions may cause pressure fluctuations that are unrelated to either the blood pressure or the holddown pressure, which in turn may result in erroneous blood pressure determinations by the sensor-based type of NIBP monitor.
Various techniques have been used to compensate the blood pressure measurement for the high motion artifacts. U.S. Pat. No. 6,132,382 entitled Non-Invasive Blood Pressure Sensor with Motion Artifact Reduction which issued Oct. 17, 2000 to Archibald et al., and U.S. Pat. No. 6,245,022 entitled Non-Invasive Blood Pressure Sensor with Motion Artifact Reduction and Constant Gain Adjustment During Pressure Pulses which issued Jun. 12, 2001 to Archibald et al., describe various techniques for providing noninvasive blood pressure monitors achieving high motion tolerance. As described in the '022 patent, for example, the effects of motion artifacts on a main source signal are reduced by the use of signals from both the main source as well as from a ring source. The main signal source is sensitive to arterial pressure, although it is also somewhat sensitive to holddown pressure. The ring signal source is mainly sensitive to holddown pressure. Both signal sources are sensitive to motion artifacts. Part of the analysis of the waveform data includes the use of an adjusted gain that is substantially constant during pressure pulses, but that may vary from pulse to pulse. Signal values obtained from the ring source are multiplied by the adjusted gain.
The effectiveness of high motion tolerance has been proven in various products, including the Vasotrac® model AMP205A (Revision K) NIBP monitor with manual high motion tolerance, which was available from Medwave Inc. of Danvers, Mass. This Vasotrac NIBP monitor incorporated a High Motion Tolerance (“HMT”) function that used an adaptive noise canceling (“ANC”) algorithm on high-pass filtered signals from the main source and the ring source. The high pass filters generally correct small movements which may occur from slow treadmill walking speeds, ambulance and ambulatory environments, and post-operative embodiments. The ANC algorithm relies on the main-to-ring noise correlation to correct large noise levels which may occur from fast walking or running on a treadmill.
To use the HMT function on this Vasotrac NIBP monitor, monitoring is begun with HMT off. The patient is observed for at least five readings to ensure that the patient has been quiet before activating the HMT function. When HMT:ON is selected, the HMT algorithm uses the last presumably valid sweep in the prior series as a baseline. When monitoring with HMT:ON, the monitor Cycle mode should be set to “Continual.” While the HMT technique provides accurate readings when the patient is moving or being moved, and while the caregiver can perform various required activities while observing the patient for high motion, the requirement on the caregiver to interrupt his or her activities in order to select HMT:ON after a given number of readings can be inconvenient to caregivers under some circumstances.
What is desired is an improved HMT function that does not require the caregiver to interrupt his or her activities, where those activities are compatible with patient observation.